1. Field of the Invention
The present invention generally relates to liners for electrical contacts within integrated circuit chip devices and more particularly to a nitride enhanced titanium liner for a refractory metal contact formed using a chemical vapor deposition process.
2. Description of the Related Art
As the device densities of integrated circuits increase, there is a need to utilize metallurgies that can be conformally coated over the resulting steep topologies. It has been found that the family of metals commonly referred to as the “refractory metals,” (i.e., tungsten, titanium, molybdenum, nickel, etc.) can be conformally coated on substrates using low pressure chemical vapor deposition techniques (LPCVD). In these techniques, a refractory metal source gas (e.g., tungsten hexafluoride) undergoes a series of reduction reactions so as to deposit a layer of tungsten on the substrate.
One of the problems with utilizing a refractory metal such as tungsten is that it has a poor degree of adhesion to the underlying insulator (e.g., silicon oxide). One method of increasing the adhesion between the refractory metal and the underlying layers is to incorporate an intermediate material having a high degree of adhesion to both the refractory metal and the underlying materials. As disclosed in U.S. Pat. No. 5,769,475 to Cronin et al. (hereinafter “Cronin”), incorporated fully herein by reference, an intermediate layer of titanium nitride, hafnium, zirconium, niobium, vanadium, chromium or nickel will provide a high degree of adhesion between tungsten and underlying layers.
In most applications, conductive structures such as gate electrodes and interconnecting stacks should have sidewalls that are as nearly vertical as possible. To the extent that these sidewalls are not vertical, extra chip surface area is unnecessarily consumed and the electrical properties of the conductive structure are degraded. Accordingly, the intermediate layer incorporated between the refractory metal and the underlying layers should have an etch rate that approximates the etch rate of refractory metal in an anisotropic (i.e., directional) etch, such as a halogen-based reactive ion etch (RIE). It has been discovered that titanium nitride has an etch rate that is substantially equal to tungsten in a halogen-based RIE, which permits an isotropic profile to be easily achieved.
As mentioned above, to form a conformal coating of tungsten or molybdenum, chemical vapor deposition (CVD) techniques are preferred to sputtering or evaporation techniques. In CVD of tungsten, tungsten reduction is induced from a tungsten hexafluoride (WF6) source gas. As a consequence, tungsten crystals form and grow on the underlying layers. The ability of these crystals to form and grow (i.e., nucleate) on the underlying layers is essential to providing a uniform film. Cronin discloses that titanium nitride present good nucleation sites for CVD tungsten or molybdenum.
Cronin also discloses that titanium nitride provides good barrier properties when incorporated in a tungsten-titanium nitride stack. In the case where tungsten is used as an interconnect structure or as a wiring plane, titanium nitride provides sufficient resistance against electromigration. Moreover, titanium nitride prevents diffusion of species (e.g., silicon) from the underlying layers into the tungsten at processing temperatures of up to 1000° C. Also, titanium nitride serves as a good barrier against fluorine penetration during CVD of tungsten. In addition, a tungsten-titanium nitride conductive stack provides an extremely low contact resistance.